The Red Devils have reportedly splashed out £57.6m, including bonuses, for the 19-year-old starlet.

But Merson thinks Louis van Gaal is taking a big risk with the youngster.

And the ex-England midfielder thinks United should question why Arsenal didn't swoop for Martial.

Merson said: "I look at him and Arsene Wenger knows that scene very, very well. I think he knows that scene better than anybody in world football.

"And he didn't go for him. I'd be interested to hear what Arsene Wenger thinks of this lad - his knowledge of French football is second to none."

GETTY

STARLET: Man United have snapped up Anthony Martial

“If you have a couple of bad games you never get it back. I think this lad has to start very, very well”

Paul Merson

When quizzed about how much pressure the France U21 star is under, Merson added: "Loads. When you sign for Man United, you have to start well.

"If you have a couple of bad games you never get it back. I think this lad has to start very, very well.

"For someone who's been playing in Monaco in front of 5-6,000 people every week, and then you have to go to play in front of 70,000 fans and you're the most expensive teenager of all time - it's a lot of pressure.

"I feel for him. I think it should have been an undisclosed fee. [The price-tag] puts too much pressure on the lad.